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p i 

J 7 





Chairman, Special Committee on Colloids, Division of Chemistry 

and Chemical Technology, National Research Council 

Member Amer. Institute Chemical Engineers 







COPYRIGHT, .1919, 





THIS little book is the result of an attempt 
to compress within a very limited space, the 
most important general properties of colloids, 
and some of the practical applications of col- 
loid chemistry. Its object will be accomplished 
if it is helpful in extending the sphere of in- 
terest in this fascinating twilight zone between 
physics and chemistry. 

J. A. 

Nov. 1, 1918. 















Although many facts and principles con- 
cerning colloids have from time immemorial 
been known and utilized empirically, the 
scientific foundation of modern colloid chemis- 
try was laid by an Englishman, Thomas Gra- 
ham, F.R.S., Master of the Mint. In two 
basic papers on this subject, the first entitled 
"Liquid Diffusion Applied to Analysis," read 
before the Royal Society of London, June 13, 
1861, the second entitled "On the Properties 
of Colloidal Silicic Acid and other Analogous 
Colloidal Substances," published in the Pro- 
ceedings of the Royal Society, June 16, 1864, 
Graham pointed out the essential facts regard- 
ing colloids and the colloidal condition, and 
established much of the nomenclature in use 

at the present day. In the first of these 



papers Graham says: "The property of vola- 
tility, possessed in various degrees by so many 
substances, affords invaluable means of separa- 
tion, as is seen in the ever-recurring processes 
of evaporation and distillation. So similar in 
character to volatility is the diffusive power 
possessed by all liquid substances, that we 
may fairly reckon upon a class of analogous 
analytical resources to arise from it. The 
range also in the degree of diffusive mobility 
exhibited by different substances appears to be 
as wide as the scale of vapor tensions. Thus 
hydrate of potash may be said to possess double 
the velocity of diffusion of sulphate of potash, 
and sulphate jti potash again double the 
velocity of sugar, alcohol and sulphate of 
magnesia. But the substances named belong 
all, as regards diffusion, to the more "vola- 
tile " class. The comparatively "fixed " class, 
as regards diffusion, is represented by a differ- 
ent order of chemical substances, marked out 
by the absence of the power to crystallize, 
which are slow in the extreme. Among the 
latter are hydrated silicic acid, hydrated alu- 
mina and other metallic peroxids of the 
aluminous class, when they exist in the soluble 


form; with starch, dextrin and the gums, 
caramel, tannin, albumen, gelatin, vegetable 
and animal extractive matters. Low diffusi- 
bility is noj^the only property which the bodies 
last enumerated possess in common. They are 
distinguished by the gelatinous character of 
their hydrates. Although often largely soluble 
in water, they are held in solution by a most 
feeble force. They appear singularly inert in 
the capacity of acids and bases, and in all the 
ordinary chemical relations. But, on the other 
hand, their peculiar physical aggregation with 
the chemical indifference referred to appears 
to be required in substances that can intervene 
in the organic processes of life. The plastic 
elements of the animal body are found in this 
class. As gelatin appears to be its type, it is 
proposed to designate -substances of this class 
as colloids, and to speak of their peculiar form 
of aggregation as the colloidal condition of 
matter. Opposed to the colloidal is the crys- 
talline condition. Substances affecting the 
latter form will be classed as crystalloids. The 
distinction is no doubt one of intimate molec- 
ular constitution. 

" Although chemically inert in the ordinary 


sense, colloids possess a compensating activity 
of their own, arising out of their physical 
properties. While the rigidity of the crystal- 
line structure shuts out external impressions, 
the softness of the gelatinous colloid partakes 
of fluidity, and enables the colloid to become 
a medium for liquid diffusion, like water itself. 
The same penetrability appears to take the 
form of cementation in such colloids as can 
exist at high temperature. Hence a wide 
sensibility on the part of colloids to external 
agents. Another and eminently character- 
istic quality of colloids is their mutability. 
Their existence is a continued metastasis. A 
colloid may be compared in this respect to 
water, while existing liquid at a temperature 
under its usual freezing-point, or to a super- 
saturated saline solution. Fluid colloids ap- 
pear to have always a pectous modification; 
and they often pass under the slightest 
influences from the first to the second condi- 
tion. The solution of hydrated silicic acid, 
for instance, is easily obtained in a state of 
purity, but it cannot be preserved. It may 
remain fluid for days or weeks in a sealed tube, 
but is sure to gelatinize and become insoluble 


at last. Nor does the change of this colloid 
appear to stop at that point. For the mineral 
forms of silicic acid deposited from water, such 
as flint, are often found to have passed, during 
the geological ages of their existence, from the 
vitreous or colloidal into the crystalline con- 
dition. (H. Rose.) The colloidal is, in fact, 
a dynamical state of matter, the crystalloidal 
being the statical condition. The colloid 
possesses Energia. It may be looked upon 
as the probable primary source of the force 
appearing in the phenomena of vitality. To 
the gradual manner in which colloidal changes 
take place (for they always demand time as an 
element) may the characteristic protraction of 
chemico-organic changes also be referred. . . . 

"It may perhaps be allowed to me to apply 
the convenient term dialysis to the method of 
separation by diffusion through a septum of 
gelatinous matter. The most suitable of all 
substances for the dialytic septum appears to 
be the commercial material known as vegetable 
parchment, or parchment-paper. . . ." 

At the beginning of the second paper above 
referred to, Graham states: "The prevalent 
notions respecting solubility have been de- 


rived chiefly from observations on crystalline 
salts, and are very imperfectly applicable to 
the class of colloidal solutions." From this 
it may be seen that Graham appreciated the 
fact that all the laws of crystalloidal solutions 
could not be applied to colloidal solutions. 
In the case of crystalloidal solutions the dis- 
solved substance is present in a state of molec- 
ular subdivision, and, according to the ioniza- 
tion theory, is in many cases dissociated into 
ions. With colloidal solutions, on the other 
hand, we have a lesser degree of subdivision, 
and the particles in solution are larger and 
more cumbersome. As Graham remarked, 
"The inquiry suggests itself whether the 
colloid molecule may not be constituted by the 
grouping together of a number of smaller 
crystalloid molecules, and whether the basis 
of colloidality may not really be this composite 
character of the molecule." This is to-day 
the idea generally accepted. 


Colloid chemistry deals with the behavior 
and properties of matter in the colloidal con- 
dition, which, as we now know, means a certain 


very fine state of subdivision. While there 
are no sharp limitations to the size particles in 
colloidal solutions, it may in a general way be 
stated that their sphere begins with dimensions 
somewhat smaller than a wave length of light, 
and extends downward well into dimensions 
which theory ascribes to the molecules of 
crystalloids. (See Table II, p. 12.) 


With the aid of the ultramicroscope, which 
renders visible particles approaching in mi- 
nuteness molecular dimensions, Zsigmondy has 
shown that there is no sharp line of demarca- 
tion between suspensions and solutions, but 
that with increasing fineness in the subdivision 
of the dissolved substance, there is a progres- 
sive change in the properties of the resulting 
fluids, the influence of gravity gradually 
yielding to that of the electric charge of 
particles, of surface tension and of other forms 
of energy. Thus in the case of metallic gold, 
subdivisions whose particles are 1 M and over 
act as real suspensions and deposit their gold, 
whereas much finer subdivisions (60 AM and 
under) exhibit all the properties of metal 


hydrosols or colloidal solutions. In the ultra- 
microscope the coarser subdivisions show the 
well-known Brownian movement, which 
greatly increases as the particles become- 
smaller, until at the present limit of ultra- 
microscopic visibility (about 5 MM) it becomes 
enormous both in speed and amplitude. 

On the other hand, there is no sharp dis- 
tinction between colloids and crystalloids, but 
as the particles in solution become smaller and 
smaller, the optical heterogeneity decreases 
correspondingly, finally vanishing as molec- 
ular dimensions are approached.* That even 
crystalloid solutions are not in a strict sense 
homogeneous, is indicated by an experiment 
of van Calcar and Lobry de Bruyn (Rec. Trav. 

* In an article entitled "Pedetic Motion in Relation to 
Colloidal Solutions " published in Chemical News, 1892, Vol. 
65, p. 90, William Ramsay, Ph.D., F.R.S, (afterward Sir 
William Ramsay), clearly expressed this view in the fol- 
lowing words: "I am disposed to conclude that solution 
is nothing but subdivision and admixture, owing to attrac- 
tions between solvent and dissolved substance accompanied 
by pedetic motion; that the true osmotic pressure has, 
probably, never been measured; and that a continuous pas- 
sage can be traced between visible particles in suspension 
and matter in solution; that, in the words of the old adage, 
Natura nihil Jti per sattum." 


chim. Pays-Bas, 1904, 23, 218), who caused 
the crystallization of a considerable part of 
saturated crystalloid solutions at the periph- 
ery of a rapidly rotating centrifuge. 

Classification of Colloids 

The broadest classification of colloids is that 
of Wolfgang Ostwald (Koll. Zeitschr., Vol. 1, 
page 291), who grouped them according to the 
physical state (gaseous, liquid or solid) of the 
subdivided substance (dispersed phase), and 
of the medium in which the particles of the 
subdivided substance are distributed (disper- 
sion medium).* Table I (page 11) shows the 
nine resulting groups and gives some instances 
of each. 

Ostwald's classification, however, is more 
theoretical than practical, for the properties of 
colloids are dependent mainly upon the specific 
nature of the dispersed substance and its degree 
of subdivision. Following Hardy, Zsigmondy 
divided colloids into two classes, the reversible 
and irreversible; the former redissolve after 

* G. Bredig proposed to call colloids "microheterogeneous 
systems." W. Ostwald called them "dispersed heterogeneous 
systems," which expression was contracted by P. P. von 
Weimarn into the term "dispersoids." 




desiccation at ordinary temperatures, whereas 
the latter do not. 







No example, since gases are miscible in all 




Fine foam, gas in beer. 



Gaseous inclusions in minerals (meer- 

schaum, pumice), hydrogen in iron, oxy- 

gen in silver. 



Atmospheric fog, clouds, gases at critical 




Emulsions of oil in water, cream, colloidal 

.LJ114 UJ.U. . ...... 

water in chloroform. 



Mercury in ointments, water in paraffin wax, 

liquid inclusions in minerals. 



Cosmic dust, smoke, condensing vapors, 

(ammonium chlorid). 



Colloidal gold, colloidal sodium chlorid, col- 

loidal ice in chloroform. 



Solid solutions, colloidal gold in ruby glass, 

coloring matter in gems. 

Table II, taken from Zsigmondy,* illustrates 
this classification, and shows how colloids hav- 
ing the same particle size or degree of sub- 
division may nevertheless act quite differently 
because of specific differences in the nature of 
the dispersed substances. 

* Colloids and the Ultramicroscope, J. Wiley & Son, Inc. 
(Translation by J. Alexander.) 


With the reversible colloids (gelatin, gum 
arabic, albumen), there is a more intimate 
union between the two phases; in fact it is 
probable that with them we have really a 
mixture of (1) a dispersed phase of water sub- 
divided in the solid, with (2) a dispersing 
phase of the solid finely subdivided in water. 
The former are therefore called emulsoids and 
the latter suspensoids. Colloids of the rever- 
sible type are also said to be hydrophile or 
lyophile, while the irreversible colloids are 
hydrophobe or lyophobe. 

No sharp line is to be drawn, however, for 
besides intermediate or transition cases be- 
tween the two classes, there may be recognized 
two groups of irreversible colloids, roughly 
defined by their behavior upon concentration: 

First: The completely irreversible , which 
coagulate while still quite dilute and separate 
sharply from the solvent with the formation of 
a pulverulent precipitate rather than a gel 
(i.e., pure colloidal metals). Chemical or 
electrical energy is needed to bring them back 
again into colloidal solution. 

Second: The incompletely reversible which, 
when quite concentrated, form a gel that may 


be easily redissolved or peptisized by com- 
paratively small amounts of reagents, unless 
the evaporation has proceeded too far (i.e., 
colloidal stannic acid). 

Consequences of Subdivision 

As the subdivision of a substance proceeds, 
the area of its effective surface increases enor- 
mously, as maybe seen from the following Table 
III adapted from Ostwald. Consequently sur- 
face forces, such as adsorption, capillarity and 
surface tension, become enormously magnified 
and of primary importance. Furthermore, the 
so-called radius of molecular attraction (p = 
50 MM) is well within the colloidal field, so that 
the specific attractive forces of the particles 
also enter as a controlling factor. In fact, 
before substances can unite chemically their 
particles must be first brought into proper 
subdivision and proximity,* by solution, fusion, 
ionization or even by mere pressure, as was 
demonstrated by W. Spring, who caused fine 

* It is a striking fact that absolutely dry sodium is not 
attacked by absolutely dry chlorin. M. Raffo and A. Pieroni 
observed that colloidal- suphur reduced silver salts energeti- 
cally, whereas even fine precipitated sulphur did not form 
silver sulphid in the cold, and did so only partially upon boiling. 




d .* 

.3 g .. C 

S * 8 1 S % s - 8 

2 8 - - -' = - 


<M C4 

a a a 

a s s a 

g* i s 1 I 

3 1 

.a .a .s .s 1 1 ; 

* a * 



-* o 


dry powders to combine chemically by high 
pressure. If the degree of subdivision is not 
profound enough to permit of the combination 
of isolated atoms or ions with each other, 
chemical combination in the strict sense may 
not occur, but there may be produced " ad- 
sorption compounds " resulting from the union 
of atomic or ionic mobs in indefinite or non- 
stoichiometric proportions, under the influence 
of more or less modified chemical forces. The 
combination of arsenious acid and ferric oxid 
which Bunsen regarded as a basic ferric arsen- 
ite, 4 Fe 2 0s, A^Oa, 5 H 2 0, has been shown by 
Biltz and Behre to be an adsorption compound; 
and Zsigmondy proved "Purple of Cassius" to be 
an adsorption compound of colloidal gold and 
colloidal stannic acid by actually synthesizing it 
by mixing the two separate colloidal solutions. 
The effect of increasing subdivision upon the 
particles in colloidal solutions is illustrated in 
Table IV, adapted from Zsigmondy. Tables 
V and VI were prepared by Zsigmondy to 
illustrate visually the relation of the sizes of 
colloidal particles to well-known microscopic 
objects on the one hand and to the theoretical 
sizes of molecules on the other. 


A. Human blood corpuscles (diameter 7.5 n, thickness 1.6 p). 

B. Fragment of rice starch granule (according to v. Hohnel) 3-8 /*. 

C. Particles in a kaolin suspension. 

E. Anthrax bacillus (length 4-15 n, width about 1 /*). 

F. Cocci (diameter about 0.5-1 n, rarely 2 /*) 

f, g, h. Particles of colloidal gold solutions Au 73a , Au^, Au ( 0.006-0.015 ft), 
i, k, 1. Particles from settled gold suspensions (0.075-0.2 n). 


a b 

? D m S 

e ei e 2 f g 


a-d. Hypothetical Molecular Dimensions 

a. Hydrogen molecule dia. 0.1 nil. 

b. Alcohol molecule dia. 0.5 MM- 

c. Chloroform molecule dia. 0.8 pp. 

d. Molecule of soluble starch dia. about 5 MM 

e-h. Gold Particles in Colloidal Gold Solutions 

e. Gold particle in Aui (too small to determine), 
d. " " " " , about 1.7 MM- 

ej. " " " " , " 3.0 MM- 
/. " " Au 73a , 6 MM- 

g. " " " Au92, " 10 MM- 
h. " " " AUOT, " 15 MM- 
u Gold particle in settled gold suspension. 








I 1 11 

II I 1 If 

l! 11 I 


3 3 ^3 03 

fi( O* o f^ 

! 1 

II i i 
os I I 

a 1 

'111 E S " " 


The Ultramicroscope 

As this instrument revolutionized colloid 
research, a brief description of it is essential. 

It is a matter of every-day experience that the 
unseen motes and dust particles in the air be- 
come visible in a beam of bright light, espe- 
cially against a dark ground, and in this simple 
fact lies the principle of the ultramicroscope. 

Faraday and later Tyndall made use of a 
convergent beam of light to demonstrate the 
optical inhomogeneity of solutions; for in 
fluids not optically clear, the path of the beam 
becomes more or less distinctly visible, because 
of the light scattered by the particles present. 
In this manner can be recognized much smaller 
quantities of matter than by spectrum analysis 
in fact less than 10~ 8 mg. (1/10,000,000) of 
metallic gold can thus be detected with the 
naked eye. 

Prof. Richard Zsigmondy while experiment- 
ing with colloidal solutions conceived the idea 
of examining this light cone microscopically. 



His preliminary experiments having demon- 
strated that he could thus see the individual 
particles in various hydrosols, he sought the 
assistance of Dr. H. Siedentopf, scientific 
director of the Carl Zeiss factory, in Jena, 
where was produced the first efficient ultra- 

The ultramicroscope consists essentially of 
a compound microscope arranged for examin- 
ing in a dark field an intense convergent beam 
of light cast within or upon the substance 
under examination. The light seen by the 
eye represents, therefore, the light diffracted, 
scattered or reflected upward by the substance 
or by particles within it. 

If within a thin beam of light from a pro- 
jection lantern we scatter successively powders 
of different substances in various degrees of 
fineness (mica ground to pass 60, 100 and 160 
mesh; lampblack; powdered oxid of zinc; 
flake and powdered graphite), some of them 
will produce only a homogeneous illumination 
of the beam in which no isolated particles can 
be seen, whereas with others, the individual 
particles are distinctly visible. 

Passing the beam through a beaker of dis- 


tilled water, nothing can be seen; but upon 
the addition of a faw drops of colloidal gold 
solution, which appears quite clear to trans- 
mitted light, the path of the beam through the 
fluid immediately becomes visible. This Tyn- 
dall effect,* as it is called, might be considered 
a criterion of colloidal solution were it not that 
very minute traces of colloidal impurities can 
produce it and it is often exhibited by solu- 
tions generally regarded as crystalloidal 
those of many dyestuffs for example; further- 
more with increasing fineness of subdivision 
the Tyndall effect decreases, disappearing as 
molecular dimensions are approached. 

Just as in the cosmic field our most powerful 
telescopes fail to resolve the fixed stars, which 
are nevertheless visible as points of light of 
varying brilliancy, so, too, in the ultramicro- 
scopic field, we can see particles much smaller 
than the resolving power of the microscope 
(that is, smaller than a wave length of light) 
provided only that they diffract sufficient light 
to affect the retina. Based upon the experi- 
ence of astronomers we may be able greatly to 
increase the sensitiveness of the ultramicro- 

* Also known as the Faraday-Tyndall effect. 


scope by fortifying the eye, so to speak, with 
the photographic plate, using at the same time 
tropical sunlight or ultraviolet light for illu- 

In the original form of the ultramicroscope, 
as perfected by Siedentopf and Zsigmondy, 
which is the one best adapted for the examina- 
tion of transparent solids, a side illumination 
is effected by a microscope objective with 
micrometer movements, which throws an 
intense but minute conical beam of light into 
the fluid contained in a little cell having 
quartz windows at the side and top. Above 
this cell a compound microscope is adjusted 
vertically, so that the narrowest part of the 
light cone occupies the center of the focal 
plane. If the fluid under examination is op- 
tically clear or if it contains particles so small 
that they cannot diffract sufficient light to 
create a visual impression, the light cone 
cannot be seen. If enough light is diffracted, 
the light cone becomes visible, being homo- 
geneous if the particles are too small or too 
close together to be individually seen, and 
heterogeneous if the particles can be individu- 
ally distinguished. Particles or dimensions 


beyond the resolving power of the microscope 
(about J At) are for brevity termed ultrami- 
crons. Ultramicrons that can individually be 
made visible are called submicrons (or hypo- 
microns) while those so small that they 
produce an unresolvable light cone are termed 

Knowing the percentage of gold present in 
a colloidal gold solution and assuming a certain 
specific gravity and uniform shape for the gold 
particles, the average size and mass of a single 
particle of colloidal gold can be calculated, if 
the number present in a given volume be first 
counted. In this manner Zsigmondy has 
shown that the smallest particles of colloidal 
gold which can be individually distinguished 
with bright sunlight, are approximately 5 /*/* 
in diameter, that is, five-millionths of a milli- 
meter; still smaller particles exist but they 
produce only an unresolvable light cone. 
Magnified 1,000,000 times such a tiny gold 
particle would be about J inch in diameter, 
while a human red blood corpuscle would be 
about 25 feet across, and a hydrogen molecule 
a speck barely visible. The gold particles in 
the unresolvable light cone must therefore 


closely approach molecular dimensions. In 
fact, by allowing amicrons to grow into visi- 
bility in a suitable solution and then counting 
them, Zsigmondy has recently shown that 
some of the particles of colloidal gold have 
a mass of 1-5. 10~ 16 mg., indicating a size of 1.7 
to 3 jjifji. 

Various other types of ultramicroscopes, 
mainly modifications of dark field illumination, 
have been developed by Cotton and Mouton, 
Ignatowski (made by Leitz), Siedentopf (car- 
dioid condenser, made by Zeiss) and others, 
and besides being useful in examining colloidal 
solutions, they have enabled pathologists to 
see and discover ultramicroscopic bacteria 
(spirochetes, infantile paralysis). 

Bausch & Lamb Optical Co. of Rochester, 
N. Y., are now producing a useful ultramicro- 

General Properties of Colloids 

The optical properties of colloids and their 
simulation of chemical compounds have been 
already referred to. The other general proper- 
ties of colloids may be considered under the 
following headings: 

1. Colloidal Protection. 

2. Dialysis, Ultrafiltration and Diffusion. 

3. Electric Charge and Migration. 

4. Pectization (Coagulation) and Peptiza- 


esting and important fact regarding reversible 
colloids is that they can communicate their re- 
versible property to irreversible colloids. The 
addition of gelatin (as little as 0.0001 per cent) 
to a solution of colloidal gold protects the latter 
against coagulation upon the addition of 
electrolytes, and permits it to redissolve after 
desiccation. Ultramicroscopic examination 
has shown that the gelatin does not affect the 



motility of the gold particles, thus disposing of 
the suggestion of Lobry de Bruyn that it acts 
by decreasing their motion. The idea ad- 
vanced by Miiller (Ber., 1904, 37, 11) that 
gelatin acts by increasing the viscosity and 
thus preventing the deposition of suspended 
particles is disproved by the fact that quince 
kernel gum, notwithstanding its viscosity, 
exercises no protective action,* whereas the 
small quantities of gelatin necessary to pro- 
duce this effect cannot appreciably increase 
the viscosity, and actually permit gold par- 
ticles to settle if they are large enough to do so. 

The action of reversible colloids in opposing 
group formation, is of great technical impor- 
tance, for in many cases it permits them to 
hinder, modify and even prevent coagulation, 
precipitation and crystallization. 

DIALYSIS. Colloid solutions possess a small 
but definite diffusibility through colloidal septa 
(parchment paper, bladder) as was recognized 
by Graham, who found that "tannic acid 
passes through parchment-paper about 200 
times slower than sodium chlorid; gum arabic 

* According to Zsigmondy, quince kernel gum acts as a 
protector with some substances. 


400 times slower/' Graham's original form of 
dialyzer may be made from a wide-mouthed 
bottle whose bottom has been removed.* The 
mouth is closed by a piece of bladder or parch- 
ment paper tightly bound on, the solution to 
be dialyzed is poured in, and the bottle im- 
mersed about halfway in water contained in a 
larger vessel. Most of the crystalloids diffuse 
through the membrane into the outer water, 
which should be frequently renewed, while 
most of the colloids remain in the original 
bottle, and may be thus obtained in a purified 
condition. Improved modern dialyzers con- 
sist of parchment or collodion sacs or thimbles, 
or even of whole bladders, which have the 
advantage of a larger dialyzing surface. 

ULTRAFILTRATION. H. Bechhold found that 
he could make filtering membranes of varying 
degrees of permeability by forming them from 
jellies of varying concentration. He used prin- 
cipally collodion dissolved in glacial acetic acid 
and afterward immersed in water, and gelatin 
jellies hardened in ice-cold formaldehyde. The 
jellies were formed and hardened on pieces of 
filter paper, which were supported from below 

* A lamp chimney will answer very well. 


by nickel wire cloth, and clamped between 
two flanges. The liquid to be subjected to 
ultrafiltration is introduced in the chamber 
thus formed and forced through the prepared 
septum by appropriate pressure, which may 
run up to 20 atmospheres or more and may 
be produced by a pump or by compressed gas 
(air, nitrogen or CO 2 ). Table VII (p. 28), pre- 
pared by Bechhold, shows various colloids ar- 
ranged in order of the diminishing size of 
their particles in solution, and was obtained 
by using ultrafilters of varying degrees of 
porosity or permeability. 

By means of ultrafiltration through ultra- 
filters of appropriate permeability, not only 
may colloids be separated from crystalloids, 
but colloids having particles of different sizes 
may be separated from each other. 

DIFFUSION. Diffusion through a septum 
is, of course, involved in dialysis. If, however, 
diffusion occurs into a jelly, many interest- 
ing phenomena may develop, especially if the 
jelly adsorbs arty of the diffusing substances 
or contains substances which can react with 

Owing to the enormous surface they present, 




Prussian blue. 

Platinum sol (made by Bredig's method). 

Ferric oxid hydrosol. 

Casein (in milk). 

Arsenic sulphid hydrosol. 

Colloidal gold hydrosol (Zsigmondy's No. 4, particles 

about 40 MM). 

Colloidal bismuth oxid (Paal's "Bismon"). 
Colloidal silver (Paal's "Lysargin"). 
Colloidal silver (von Heyden's "Collargol," particles about 

20 MM). 
Colloidal gold hydrosol (Zsigmondy's No. 0, particles 

about 1-4 MM). 
Gelatin solution, 1 per cent. 
Hemoglobin solution, 1 per cent (molecular weight about 


Serum albumin (molecular weight about 5000 to 15,000). 
Diphtheria toxin. 
Colloidal silicic acid. 
Lysalbinic acid. 
Deuteroalbumoses A. 

Deuteroalbumoses B (molecular weight about 2400). 
Deuteroalbumoses C. 

Dextrin (molecular weight about 965). 

colloidal gels exhibit a powerful adsorptive 
action. In fact, even when percolated through 
such a relatively coarse-grained septum as sand, 
most solutions issue with a materially reduced 


content of solute, and benzopurpurin solutions 
may be thus decolorized. Further, if a solute 
hydrolyzes into ions having different degrees 
of adsorbability or different rates of diffusibil- 
ity, they may be actually separated by diffusion 
through a colloidal gel. 

This phenomenon is nicely exhibited by 
what may be termed a " patriotic test tube," 
prepared by filling a tube about two-thirds full 
with a slightly alkaline solution of agar contain- 
ing a little potassium ferrocyanid and enough 
phenolphthalsin to turn it pink. After the agar 
has set to a firm gel, a solution of ferric chlorid 
is carefully poured on top, and almost instantly 
the separation becomes evident. The iron 
forms with the ferrocyanid a slowly advancing 
band of blue, before which the more rapidly 
diffusing hydrochloric acid spreads a white 
band as it discharges the pink of the indicator. 
After the lapse of a few days the tube is about 
equally banded in red, white, and blue. 

Even then the tubes do not cease to be of 
interest, for if they are allowed to stand several 
weeks the pink color is all discharged and there 
develop peculiar bands or striations of blue, 
apparently due to the fact that the iron ferro- 


cyanid temporarily blocks the diffusion pas- 
sage, which are gradually opened again after 
a layer of the blue salt has diffused on from the 
lower surface. 

f Not only may ions be thus separated, but if 
two solutes in the same solvent possess differ- 
ent rates of diffusion or different degrees of 
adsorbability, they also may be separated from 
each other by diffusion through a colloidal gel 
or septum. (Differential Diffusion.) 

particles of practically all colloidal solutions 
possess an electric charge, and under the 
influence of an electric current (difference of 
potential) move toward the electrode having 
the opposite charge. (Electrophoresis.) In 
general, when two substances are brought into 
contact, the one having the higher dielectric 
constant becomes positively charged, whereas 
the one with the lower dielectric constant 
becomes negatively charged (Cohen's Law). 
Since water has a high dielectric constant (80), 
most substances suspended in pure water 
become negatively charged and wander to the 
anode. On the other hand if suspended in oil 
of turpentine, which has a low dielectric 


constant (2.23), they become positively charged 
and wander to the cathode. 

If, however, electrolytes are present, Coehn's 
law is superseded by other controlling factors, 
such as the adsorption of ions, which may give 
their charge to the suspended particles. In 
fact Hardy found that in pure water albumen 
was amphoteric; in the presence of a trace of 
alkali it acquired a negative charge and 
migrated to the anode; but a trace of acid gave 
it a positive charge and it then migrated to the 
cathode. The following table shows the usual 
charge and migration tendency of a number 
of aqueous colloidal solutions. 

Charged + Charged 

Migrate to Cathode (- Pole) Migrate to Anode (4- Pole) 

1. Hydrates of Fe, Cu, Cd, Al, Zr, 1. Sulphids of As, Sb, Cu, Pb, Gd. 

Ce, Th. Halides of Ag. 

2. Titanic acid. 2. Stannic acid, silicic acid. 

3. Colloidal Bi, Pb, Fe and Gu 3. Colloidal Pt, Au, Ag, and Hg, 

(Bredig's method). I, S, Se. 

4. Albumen, hemoglobin, agar. 4. Gum arabic, soluble starch, 

gamboge, mastic, oil emulsion. 

5. Basic Dyes: Methyl violet, 5. Acid Dyes: Eosin, fuchsin, 

Bismarck brown, methylen anilin blue, indigo, soluble 

blue, Hofmann violet. Prussian blue. 

stated pectization means the coagulation of a 
colloidal sol, and peptization its redispersion. 
If a small quantity of an electrolyte is added 


to a pure ruby red colloidal gold solution, the 
latter changes to a blue or violet color, and 
deposits its gold as a fine blackish coagulum 
or precipitate.* By watching in the ultra- 
microscope the coagulation of very dilute 
milk by dilute acid, the individual particles of 
the colloidal casein may be seen to gather 
gradually together into groups, whose motion 
becomes progressively less as their size in- 
creases, until they are no longer able to stay 
afloat, and finally coagulate in large grape-like 
clusters. Hardy believes that the particles of 
colloids adsorb the oppositely charged ions of 
electrolytes present; at the isoelectric point 
(that is when there is no excess either of posi- 
tive or negative charges on the particles) coag- 
ulation occurs. If, however, an excess of elec- 
trolyte be added all at once, the isoelectric 
point may be passed before coagulation occurs, 
and the particles acquire a charge opposite to 
the one they had originally. Under such con- 
ditions, no coagulation may result. 

* The amount in milligrams of protective colloid just 
sufficient to prevent the change to violet of 10 cc. of bright 
red colloidal gold solution by the addition of 1 cc. of a 10 
per cent solution of NaCl, is called the "gold figure" or "gold 
number" of the protector. 


Burton epitomizes the difference in action 
of various electrolytes as follows: "Two 
remarkable results are evident on comparing 
the coagulative powers of various electrolytes 
on colloids of different kinds; first, the coagu- 
lation depends entirely on the ion bearing a 
charge of sign opposite to that of the colloidal 
particle; and, second, with solutions of salts, 
trivalent ions have, in general, immensely 
greater coagulative power than divalent ions, 
and the latter, in turn, much greater than 
univalent. Acids and alkalis in particular 
cases act more strongly than the corresponding 

High-tension electric discharges may also 
effect the coagulation or precipitation of a 
finely subdivided or dispersed phase; which 
fact was utilized by Sir Oliver Lodge in dis- 
pelling fogs, and by Cottrell hi coagulating 
smelter and similar fumes. 

PEPTIZATION. So strong is the analogy 
between digestion and colloidal disintegration 
that Thomas Graham, the father of colloid 
chemistry, coined the word peptization to 
express the liquefaction of a gel. He first 
speaks of the coagulation or pectization of 


colloids. "The pectization of liquid silicic 
acid," he states, "and many other liquid 
colloids is effected by contact with minute 
quantities of salts in a way which is not under- 
stood. On the other hand, the gelatinous acid 
may be again liquefied, and have its energy 
restored by contact with very moderate 
amounts of alkali. The latter change is 
gradual, 1 part of caustic soda, dissolved in 
10,000 water, liquefying 200 parts of silicic 
acid (estimated dry) in 60 minutes at 100 
degrees. Gelatinous stannic acid also is easily 
liquefied by a small proportion of alkali, even 
at the ordinary temperature. The alkali, too, 
after liquefying the gelatinous colloid, may be 
separated again from it by diffusion into water 
upon a dialyzer. The solution of these col- 
loids in such circumstances may be looked 
upon as analogous to the solution of insoluble 
organic colloids witnessed in animal digestion, 
with the difference that the solvent fluid here 
is not acid but alkaline. Liquid silicic acid 
may be represented as the 'peptone' of 
gelatinous silicic acid; and the liquefaction of 
the latter by a trace of alkali may be spoken of 
as the peptization of the jelly. The pure 


jellies of alumina, peroxide of iron and titanic 
acid, prepared by dialysis, are assimilated 
more closely to albumen, being peptized by 
minute quantities of hydrochloric acid." 

Peptization is in reality deflocculation, a 
dispersion of groups into separate particles 
which once more acquire active motion and 
remain afloat or in solution. The detergent 
action of soap and dilute alkalis is due to the 
fact that they deflocculate adhering particles 
of " dirt." 


Practical Applications of Colloid Chemical 

The practical applications of colloid chemis- 
try are so manifold and widespread that they 
touch every branch of science and technology. 
Whole books may be and have been written on 
many of the most restricted fields, while the 
scientific literature teems with monographs 
and articles, directly on, or applicable to, 
colloid-chemical subjects. In what follows, it 
will be possible therefore to give not an ex- 
haustive, but only a most general survey, 
intended rather to show the ubiquity of col- 
loid phenomena; and many important topics 
must be dismissed with a most rudimentary 
discussion, altogether incommensurate with 
theu* importance. 

ASTRONOMY. As matter in colloidal state 
is so common on our relatively minute earth, it 
is but natural to expect to find many instances 
of colloidal dispersion in the immensity of the 



Cosmic dust Is widely distributed throughout 
space, and as it is gathered up by the superior 
attraction of the larger heavenly masses (suns, 
planets, etc.), which in any system grow at the 
expense of the smaller masses, fresh quantities 
are continually produced by the collisions of 
bodies in space, as well as the disintegration 
of meteorites, comets, asteroids, etc. 

The tails of comets seem to consist almost 
entirely, and the nuclei and coma largely, of 
colloidally dispersed matter. The great comet 
of 1882 which made a transit of the sun, was 
invisible against the solar disc (a position corre- 
sponding to attempted observation of colloidal 
particles in the ordinary microscope against a 
luminous background), but became visible 
again after passing beyond the sun's disc (a 
position corresponding to successful observa- 
tion of the same colloidal particles in the ultra- 
microscope against a dark background, the eye 
of the observer being protected from the source 
of illumination). 

The streaming of the cometary tails away 
from the sun may be due to the ionization of 
the constituent colloidal particles, and their 
consequent electrical repulsion; or more prob- 


ably, it may be due to the sun's rays, as was 
pointed out by J. Clerk Maxwell. The inten- 
sity of the action of the sun's rays on a particle 
depends upon its surface, which varies as the 
square of its diameter, whereas the gravitation 
of the same particle to the sun depends upon its 
mass, which varies as the cube of its diameter. 
Theoretically in the case of a particle whose 
density equals that of water, the repulsion 
balances gravitation when the diameter reaches 
0.0015 mm. (= 1.5 /*). As the diameter di- 
minishes the repulsive force gains the ascend- 
ancy, soon reaching a maximum and again 
diminishing, until when the particle has a 
diameter of only 0.00007 (= 70 MM) the two 
forces again balance each other.* 

These figures, which refer to a substance hav- 
ing the density of water, are approximately of 
colloidal dimensions; but in the case of denser 
bodies the subdivision would be even more 
profound. It is therefore not surprising that, 
when the earth recently passed through the tail 
of a comet, no disturbance of any kind was 

* See Simon Newcomb's article on "Comet," Encyclo- 
pedia Britannica, llth edition. Also Svante Arrhenius, 
" Worlds in the Making," Harper & Bros. 


noticed. The comet's tail is a vast celestial 
camouflage its luminosity a macroscopic Far- 
aday-Tyndall effect. 

The nebulae, too, apparently consist of finely 
dispersed matter, rendered luminous by neigh- 
boring suns; although with them as with the 
comets, a small part of the light may result 
from self -luminescence (incandescent gas, etc.). 

METEOROLOGY. What we commonly call 
" weather conditions " are largely dependent 
upon the degree of dispersion of water in the 
atmosphere, and this dispersion is mainly 
effected and maintained by solar heat and 
electrical energy. When air carrying water 
vapor is chilled by rising to a higher level, 
meeting a colder mass of air, or even by the 
alternation of night and day, the moisture 
it contains assumes the colloidal state as cloud, 
fog or mist; and as the coagulation of the 
dispersed water proceeds, these in turn may 
condense still further into dew, rain, snow or 
hail, depending upon conditions. When the 
dispersed water aggregates, there is naturally 
set free the energy originally used in its disper- 
sion, and this may appear as electricity (light- 
ning) especially if the aggregation occurs 


suddenly as is the case in thunder and hail 
storms. We have all noticed how a nearby 
lightning flash is promptly followed by an 
increased fall of raindrops. 

Were it not for our atmosphere, the sun would 
appear to us like a fiery ball set in a black star- 
sprinkled sky. The blue color of the sky is due 
to diffraction of the sunlight by the earth's 
atmosphere, a gigantic Tyndall effect. If we 
look edgewise through a clear sheet of glass, we 
at once notice the green color due to colloidally 
dispersed iron, and in like manner, if we look 
through a great length of the atmosphere the 
prevailing color is blue. As the poet Campbell 
beautifully puts it : 

!< Tis distance lends enchantment to the view, 
And robes the mountain in its azure hue." 

After the tremendous explosive eruption of 
the volcano Krakatoa in 1883, colloidal dust 
and ashes were projected so high that they 
gradually spread around the earth, causing 
" golden sunsets." 

We do not know to what extent electrical 
conditions on the earth affect the dispersion of 
substances in its atmosphere; but since half 
of the earth is always heated by the sun while 


the other half is cooler, thermoelectric currents 
are continually circulating about the earth. 
Variations in solar radiation due to sun-spots 
and the like, cause violent electric and magnetic 
storms which are intimately connected with the 
aurora, and other atmospheric phenomena 
(ionization, electrical charge of dispersed par- 
ticles); and it is well known that sun-spots 
exercise a potent influence on the weather. 

nary properties of the solid constituents of the 
earth's crust depend more upon their state of 
physical subdivision than upon their chemical 
constitution. Atterberg classified the frag- 
ments of minerals and rocks as follows: 


Boulders 2 m. to 20 cm. 

Pebbles 20 cm. to 2 cm. 

Gravel 2 cm. to 2 mm. 

Sand 2 mm. to 0.2 mm. 

Earth 0.2 mm. to 0.02 mm. 

Loam 0.2 mm. to 0.002 mm. 

Clay smaller than 0.002 mm. 

The smaller the particles, the greater their 
capillarity and the ease with which they are 
moved by wind and by water, but the less 
their permeability to water. Fine defloccu- 


lated clay is carried thousands of miles by 
rivers until it is finally coagulated by the salts 
of the ocean, as may be observed in the deltas 
of the Ganges, Nile and Mississippi. Fine 
particles are easily cemented by pressure or 
igneous action into rocks (e.g., sandstone, 
slate), or may act as a cement for large par- 
ticles (e.g., pudding-stone) or as a matrix for 

Many minerals are themselves colloidal gels 
(e.g., opal, flint, bauxite) or result from the 
weathering of other minerals with consequent 
gel formation (e.g., kaolin from kaolinite, 
serpentine from diabase). Most gems owe 
their colors to impurities colloidally dispersed 
within them (e.g., ruby, emerald, amethyst). 
Dendrites are formed by solutions diffusing 
through mineral gels. Colloidal minerals usu- 
ally adsorb, and are dyed by aniline dyes 
(methylen blue), whereas crystalloid minerals 
are unaffected. 

CLAY AND CEEAMICS. The effect of vege- 
table extractive matters on the working 
properties of clay have been known from 
ancient times in the Bible (Exodus V) it 
is mentioned that brick cannot be made with- 


out straw. Recently patents have been taken 
out for " Egyptianizing " clay by adding to it 
tannin, extract of straw, humus and the like. 
Glue and similar protective colloids defloccu- 
late or "free out " clay and make it " cover " 
in paper-coating and kalsomining. The work- 
ing properties of clays depend largely upon the 
size of their constituent particles and their 
state of aggregation. This is especially evi- 
dent in ceramics. Articles molded of clay and 
then burned, lose their hydrosol condition and 
become hardened into pottery. 

AGRICULTURE. Although from time im- 
memorial farmers have classified soils on the 
basis of their physical and physiological 
character as "light " or "heavy," "rich " or 
"poor," "productive" or "unproductive," 
etc., it is only within comparatively recent 
years that chemists have begun to realize the 
full importance of the role played by the 
colloids, especially the organic colloids of the 

Many important properties of soils, such as 
permeability, capillarity, absorption, moisture 
content, etc., are dependent not so much upon 
the chemical composition as upon the size of 


the constituent soil particles. (See Atterberg, 
Schwed, landw. Akad., 1903, and Chem. Zeit.. 
1905, 29, 195; Patten and Waggaman, U. S. 
Dept. of Agri. Bureau of Soils, Bull. No. 52, 
1908). In coarse sand, for example, the 
amount of water is greatest at the bottom and 
smallest at the top, whereas in fine clay the 
distribution is much more uniform. 

Among the natural agencies tending to 
increase the size of the minute soil particles 
may be mentioned heat with its drying or 
evaporative effect, freezing, and the coagulat- 
ing or flocculating action of soluble inorganic 
salts and some organic substances present in 
the soil. On the other hand, included in that 
little known class of substances vaguely de- 
scribed as " humus," there are numerous 
organic substances derived from the bacterial, 
plant, or animal debris, or exuded by the roots 
of plants, which act as protective colloids 
(Schutzkolloide) and tend to produce and 
maintain the hydrosol, or deflocculatd con- 
dition. (See P. Ehrenberg, "Die Kollide des 
Ackerbodens," Zeits. angew. Chem., 1908, 
41, 2122.) In an excellent paper on the 
mechanics of soil moisture, L. J. Briggs (U. S. 


Dept. of Agric., Bureau of Soils, Bull. No. 
10, 1897) pointed out that very small quantities 
of certain organic substances, such as are con- 
tinually being produced in the soil by the 
decay of organic matter, greatly decrease the 
surface tension of solutions, thus counteracting 
to a large extent the effects of the surface 
application of soluble salts which would tend 
to draw moisture to the surface by increasing 
the surface tension of the capillary water of 
soils. It is well known, however, that an 
excess of salts will ruin a soil physically, as is 
evident after flooding by sea water or the 
excessive application of chemical fertilizers. 
Of interest in this connection is the recent work 
of the Bureau of Soils, U. S. Department 
of Agriculture, carried out by Cameron, 
Schreiner, Livingston and their co-workers. 
Thus plants grown in the unproductive Ta- 
koma soil, were greatly benefitted by green 
manure, oak leaves, tannin and pyrogallol. 
The injurious effects of quinone and some 
other organic substances may be due to their 
ability to precipitate or flocculate the pro- 
tective colloids of the soil; for as Lumiere 
and Seyewetz have shown (Bull. Soc. Chim., 


1907, 4, 428-431; J. S. C. L, 1907, 703) 
quinone renders gelatin insoluble. 

The fact observed by Fickenday (J. Landw., 
1906, 64, 343) that more alkali is required to 
flocculate natural clay soils than kaolin sus- 
pensions, he attributes to the protective action 
of the humus present (see Keppeler and Spang- 
enberg, J. Landw., 1907, 55, 299). 

A. S. Cushman, in his excellent work upon 
the use of feldspathic rock as fertilizer (U. S. 
Dept. of Agriculture, Bureau of Plant Indus- 
try, Bulletin No. 104; Cushman and Hubbard, 
J. Am. Chem. Soc., 30, 779), has shown that 
the fine grinding of feldspar increases the 
amount of potash available under the action of 
water. Thus, a coarse powder having an 
area of 43 sq. cm. per cc. of solid feldspar 
yielded 0.013 per cent, whereas a fine powder 
whose area was 501,486 sq. cm. per cc. yielded 
0.873 per cent of potash and soda. These fine 
particles averaged about 0.1 /* in diameter, 
which is relatively large as compared with 
colloidal dimensions; but under the action of 
physical and chemical soil agencies they 
undergo further disintegration, finally reaching 
a colloidal condition in which still more of 


their potash is available, a condition favored 
and maintained by the organic protective 
colloids of the soil. 

With these brief and inadequate remarks we 
must dismiss this subject of such vast impor- 
tance and fascinating interest, referring to the 
extensive literature, much of which is quoted 
in Bulletin No. 52 and the other publications 
of the Bureau of Soils. 

METALS. The addition of protective colloids 
to electroplating baths tends to the production 
of fine-grained non-crystalline deposits. A. G. 
Betts in a paper entitled "The Phenomena of 
Metal Depositing " (J. Am. Electrochem. Soc., 

1905, 8, 63) has shown that there are many 
factors influencing the action of the colloid, 
and has suggested a number of possible ex- 
planations. The correct explanation, how- 
ever, has been given by Mtiller and Bahntje 
(Z. Elektrochem., 1906, 12, 317; J. S. C. I., 

1906, 484) who state that the added colloid 
keeps the deposited metal (copper) in an 
amorphous, non-crystalline condition, gelatin 
producing the most powerful effect, egg al- 
bumen considerably less, while gum and 


starch have comparatively little action. They 
also found that the deposited copper weighed 
about 0.2 per cent more than under normal 
conditions, indicating that some of the colloid 
had been carried down with the metal. 

The relative efficiency of the colloids just 
referred to corresponds to their relative effi- 
ciency in protecting from coagulation solutions 
of colloidal gold (see Zsigmondy, J. S. C. I., 
1902, 192; also Colloids and the Ultramicro- 
scope, p. 81), which is additional evidence 
that we have another instance of protective 
colloidal action, by which the crystallization 
forces of the metal are powerfully influenced. 

METALLURGY. Since coarsely crystalline 
metals are brittle, tending to split along the 
lines of crystal cleavage, various physical and 
chemical means are employed in technical 
practice to obtain a hard, fine-grained struct- 
ure. (See I. Langmuir, Iron & Steel Inst., 
Sept. 1907; J. S. C. I., 1907, 1094.) Among 
the physical methods are chilling and rolling, 
while the chemical methods involve the re- 
moval of undesirable constituents (as in the 
conversion of pig iron into steel) or the addi- 
tion of desirable constituents (as in the case- 


hardening and the manufacturing of " chrome 
steel/' " nickel steel/ 7 etc.). For example, 
P. Putz has shown (J. S. C. L, 1907, 614) that 
the predominant effect of vanadium in steel is 
to decrease the size of the ferrite grains and 
make the material tougher; it renders the 
ordinary structure due to pearlite fine-grained 
and homogeneous (see also Beilby, Proc. Roy. 
Soc. A., 79, 463; J. S. C. L, 1907, 926). 

Now, while the question is one of very great 
complexity, many of the facts at present 
available seem to indicate that one of the 
causes favoring the fine-grained structure is 
the inhibition of crystallization by substances 
colloidally dissolved in the molten mass. Thus 
part of the carbon in iron and steel exists in the 
graphitic form, and as graphite is slightly soluble 
in iron (see C. Benedicks, Metallurgie, 1908, 
5, 41; J. S. C. L, 1908, 406); some of it will, 
under proper conditions, be found in colloidal 
form (Carnegie Research Reports, J. S. C. L, 
1908, 27, 570; F. Wust, J. S. C. L, 1907, 26, 
412; Hersey, J. S. C. L, 27, 531). Besides 
metals may dissolve each other and other 
substances colloidally, but in the case of ordi- 
nary metals this is not easy to demonstrate. 


An observation recently made by J. Alex- 
ander * is of interest here. Moissan (Comptes 
rend., 144, 593, J. S. C. L, 1907, 413) has noted 
that the addition of a little platinum to me- 
tallic mercury causes the latter to " emulsify " 
in water. Upon making up such an " emul- 
sion," Alexander noticed that the supernatant 
fluid remained turbid upon standing, and 
therefore examined the fluid in the ultrami- 
croscope, which revealed the presence of col- 
loidal metallic particles in active motion. 

DYEING. The difference between a physi- 
cal mixture and a chemical compound is 
frequently illustrated by dissolving out the 
sulphur from a mixture of iron filings and 
sulphur dust, and showing that the solvent, 
carbon bisulphid, does not affect the compound, 
ferrous sulphid. That in many cases dyeing 
is due, not to chemical combination, but to an 
adsorption f of the dye by the colloidal fiber, is 
evident from the fact that some dyestuffs can 
be extracted from the dyed fiber by means of 
alcohol. Investigation has shown that many 

* J. S. C. I., 1909, 28, 280. 

t In some cases adsorption may be followed by undoubted 
chemical combination. 


dyes are colloidal in solution, and the selective 
coloring of various fibers, tissues, cells, nuclei, 
etc., is probably due to selective adsorption or 
precipitation of one colloid by another. The 
ultramicroscopic researches of N. Gaidukov 
(Zeitsch. f. angew Chem., 21, 393) support this 

The phenomena of dyeing are rather numer- 
ous and complicated, for the dyestuffs are 
numbered by thousands, and the various fibers, 
tissues, etc., such as cotton, silk, wool, linen, 
jute and straw, all react characteristically. In 
some cases the colloid fiber adsorbs the dye, as 
with basic colors which dye silk and wool 
directly; in other cases there is necessary a 
mordant which is first adsorbed and then fixes 
the color. Certain colors mutually precipitate 
each other and may in fact serve as mordants 
for each other, e.g., methylen blue and dianil 
blue 2 R.; patent blue V and magenta. 

Colloid chemistry also throws much light 
upon many obscure points in the practical art 
of dyeing. It is possible to obtain much more 
level colors in old dye liquors than in fresh 
ones, and here it seems that colloidally dis- 
solved substances are responsible, exercising 


a restraining action upon the absorption of the 
color. The addition of Glaubers' salt facili- 
tates level dyeing, probably by its action as an 
electrolyte, producing a partial coagulation of 
the dyestuff, so that the particles of the latter, 
thereby made larger, are absorbed more slowly 
and evenly. 

SOAP. In a comprehensive paper entitled 
" Modern views on the constitution of soap " 
(see J. S. C. L, 1907, 26, 590) Lewkowitsch 
epitomizes the views of Merklen substantially 
as follows: " Commercial soap is a product 
having an essentially variable composition 
dependent upon (1) the nature of the fatty 
acids, (2) the composition of the 'nigre' (in 
the case of settled soaps), (3) the tempera- 
ture at which the boiling is conducted; it 
behaves like a colloid and should not be re- 
garded as a compound of sodium salts of fatty 
acids, with which a definite amount of water is 
combined chemically, but rather as an 'ab- 
sorption-product' whose composition is a func- 
tion of the environment in which the salts of 
the fatty acids happen to be at the moment of 
the finishing operations. " 

Merklen's views conflict with the views as to 


the chemical composition of soap previously 
advanced by Lewkowitsch, who states, in 
conclusion: "But whatever may be the out- 
come of renewed experiments, Merklen's views 
cannot fail to stimulate further research into 
the composition of soap, and thus help to raise 
the industry of soap-making, which has too 
long been looked upon as a mere art, to the 
rank of a scientifically well-founded industry, 
the operations of which are governed by the 
laws of mass action, the phase rule and the 
modern chemistry of colloids. " 

The colloidal nature of soap solutions is 
indicated by their turbidity and their gelatin- 
ization. That the detergent action of soap is 
consequent upon its deflocculating effect was 
brought out in the interesting Cantor Lecture 
of H. Jackson (J. Soc. Arts, 55, 1101 et seq.), 
who examined microscopically the supernatant 
fluid resulting from washing a dirty cloth with 
soap and water, and found in it countless 
particles in a state of oscillatory motion 
("pedesis ") When an individual fiber was 
bathed in soap solution, the dirt particles 
gradually loosened and began to oscillate; 
upon substituting salt solution for the soap, 


the particles flocculated and the motion 
ceased. An ultramicroscopic examination of 
the detergent effects produced by soap should 
prove of interest. 

In this connection mention must be made 
of the excellent paper of W. D. Richardson 
on "Transparent Soap" (J. Amer. Chem. 
Soc., 30, 414), which he terms a supercooled 
or supersaturated solution, having distinctly 
crystalline tendencies and exhibiting colloidal 
properties. Having in mind the fact that 
the salts of the higher fatty acids dissolve 
in water as colloids, and in alcohol as crystal- 
loids (S. Ya. Levites, Zeits. Chem. Ind. Kol- 
loide, 2, 208, et seq., J. S. C. L, 1908, 1134; 
Mayer, Schaeffer, and Terroine, Compt. rend., 
146, 484) and also the fact that the alcohol or 
equivalent solvents (glycerol, sugar, etc.) are 
used in transparent soap, it seems probable 
that the crystals which frequently form in it 
are due to the slow separation of such part of 
the soap as is in crystalloid solution. This 
view is supported by the fact adduced by 
Richardson (loc. cit., p. 418) that the fatty 
acids separated from the crystals had a higher 
melting point than those separated from the 


clear matrix. The isolation of the crystals 
was difficult because of their ramifying tend- 
ency, which recalls some of the crystal figures 
exhibited by some mixtures of crystalloids and 
colloids. What may be called the crystalloid 
phase of soap is apparently governed by the 
same factors as those which Tamman has 
pointed out as governing the crystallization of 
supercooled solutions, i.e., 1st, the specific power 
of crystallization; 2nd, the speed of crystalliza- 
tion; 3rd, the viscosity (see Zsigmondy, Colloids 
and the Ultramicroscope, p. 128 et seq.). Thus, 
gold ruby glass when quickly cooled (or super- 
cooled) is colorless, but acquires a red color 
upon reheating to the softening point. By 
ultramicroscopic examination Zsigmondy 
showed that the nuclei of metallic gold, which 
in the colorless glass were amicroscopic, grew 
into ultramicroscopic visibility in the red glass. 
It therefore seemed to the author that a most 
important factor in determining the trans- 
parency of transparent soap would be the 
speed of cooling, and some experiments were 
made along this line. 

A piece of commercial transparent soap was 
melted and cast into two cups, one of which 


was quickly chilled in ice, while the other was 
allowed to cool slowly by immersion in hot 
water. The quickly cooled piece was trans- 
parent, while the other was practically opaque, 
and showed upon ultramicroscopic examina- 
tion much larger ultramicrons than the trans- 
parent piece. 

After standing three or four months, the 
quickly cooled soap was still transparent to 
the naked eye, whereas large opaque spots 
could be seen in the slowly cooled piece. In 
the ultramicroscope the former appeared as 
before, whereas the latter showed large and 
perfectly resolvable crystals in a clear matrix. 

These experiments give us an inkling as to 
what occurs during the " heat treatment " and 
tempering of metals, and it is to be hoped that 
some technique may be devised that will give 
us even a clearer insight than does " etching," 
into the changes that occur in metals in metal- 
lurgical operations (heat treatment), use, age, 
and even " disease " (tin for example). 

MILK. From a colloid chemical stand- 
point, the main constituents of milk may be 
classified as follows: 



In crystalloid 

salts (such as NaCl, etc.) 
sugar (lactose). 

In colloidal C casein an unstable or irreversible colloid. 
dispersion ( lactalbumin a stable or reversible colloid. 
In suspension* milk fat. 

Most formulas and recipes for modifying 
cows' milk for infant feeding, and for that 
matter, many analyses, combine the percent- 
ages of lactalbumin and of casein under the 
collective title of " total proteids," thereby 
obscuring the highly important fact that the 
lactalbumin stabilizes and protects the casein 
from coagulation by acid and rennin.f 

The subjoined table will show how milks 
are influenced by a difference in the ratio 
between the casein and lactalbumin. 



Kind of milk. 




Behavior with 

Behavior with 



3 02 


3 64 

Readily coag- 






1 26 

3 78 

Not readily 

Not readily 





1 55 

1 64 

* It is probable that some of the fat is in colloidal dispersion, 
t See Alexander and Bullowa, Jour. Am. Med. Assoc., Vol. 
LV, p. 1196 (Oct. 1, 1910). 


It is interesting to note that the milks in the 
above table are arranged in order of their 
digestibility, which also corresponds with their 
relative colloidal protection. Thus Jacobi has 
stated that asses' milk has always been 
recognized as a refuge in digestive disorders in 
which neither mother's milk nor cow's milk or 
mixtures were tolerated. 

The addition of protective colloids to cows' 
milk stabilizes it and makes it act more like 
mother's milk when treated with acid and 
rennin. In fact, if sufficient protective colloid 
be added, coagulation of the casein in the 
stomach may be entirely prevented, or at least 
the coagula kept in a very fine state of sub- 

The action of protective colloids is beauti- 
fully illustrated in the ultramicroscope, which 
enables us to see the individual particles of 
cows' casein in active motion and watch the 
course of their coagulation by acid, first into 
small and then into larger and larger groups, 
whose motion decreases as their size increases, 
until finally they sink out of solution in coagu- 
lated masses. If, however, some gelatin or 
gum arabic solution be added to the cows' 


milk before the addition of the acid, the casein 
particles continue their active dance and do 
not coagulate. In this connection it is in- 
teresting to note that the casein particles in 
mother's milk appear to be much smaller than 
those in cow's milk, probably because of the 
more highly protective medium in which they 
are formed and exist. 

Although their method of action was not 
perfectly understood, protective colloidal sub- 
stances have for years been used in the modi- 
fication of cow's milk for infants. For over 
thirty years Jacobi has advocated the addi- 
tion of gelatin and gum arabic to cow's milk 
and infant's diet, and the use of gruels, dex- 
trinized starch and similar reversible colloids 
is familiar to all. It is interesting to note that 
sodium citrate, which is largely employed as 
an addition to cow's milk, acts as a protective 
colloid, and when going into solution actually 
exhibits actively moving ultramicrons in the 
ultramicroscope, a fact which indicates its 
colloidal condition. 

In addition to stabilizing the casein, pro- 
tective colloids in milk have a very important 
influence on the milk fat. In the first place 


is to be considered the emulsifying and 
emulsostatic action of reversible colloids. Of 
much greater importance, however, is the 
result of stabilizing the casein, for insufficiently 
protected casein in curding carries down 
mechanically most of the milk fat present, 
yielding a greasy, fatty curd which is very 
difficult for the digestive juices to dissolve. 

ICE CREAM. It is a fact well known to 
practical ice cream makers, and amply proven 
by experience, that ice cream made without 
eggs, gelatin or some similar colloidal ingredi- 
ent, is gritty, grainy or sandy, or else soon 
becomes so upon standing; whereas ice cream 
made with small quantities of colloids possesses 
that rich, mellow, velvety texture so much in 
demand. Here the added colloid acts as an 
inhibitor of crystallization or practically speak- 
ing as a preserver of texture. The added 
colloid, especially gelatin, which is the one 
most frequently used, also serves as a protective 
colloid in preventing the coagulation of casein, 
apparently an irreversible hydrosol and a 
normal constituent of ice cream. In view of 
what has been said above, it is evident that 
gelatin thus renders ice cream more digestible. 


A very misleading impression is given by 
some official food chemists referring to gelatin 
in ice cream as a " filler," which naturally leads 
to the idea that it is an inferior ingredient 
added in quantity to cheapen the product. 
But as gelatin is expensive and as but | per 
cent is used, such a view is evidently erroneous. 
The food value of gelatin as a protector of the 
body's nitrogen being generally admitted, and 
its effect in milk being very beneficial from a 
digestive point of view, its use in ice cream in 
the quantities referred to is necessary, legiti- 
mate and scientific. 

CONFECTIONARY. In gum drops, marsh- 
mallows, " moonshine " and other candies, use 
is made of gum arabic, gelatin, albumen, and 
other colloids to prevent the crystallization of 
the sugar. Thus, besides adding to the food 
value, they give the candy a smooth and 
agreeable taste, and preserve it in saleable 

BREWING. Beer contains dextrin and al- 
bumin, both colloids. In the brewing process 
many factors appear which tend to coagulate 
the albumen. The influence of solid surfaces 
is illustrated by changing the walls of the 


fermenting vessel. Thus a certain wort fer- 
mented in glass or enameled vessels showed 
0.2450 per cent of albumen; the same wort 
fermented in a paraffin-lined vessel showed 
0.1925, and in a vessel lined with pitch only 
0.1750 per cent of albumen. Old-fashioned 
brewers would never use any vessel unless it 
had first been treated with a decoction of 
malt kernels and nut leaves, or else with 
"fassgelager " (barrel dregs) which acts like 
the so-called "bierstein," a deposit consisting 
chiefly of organic substances that forms upon 
new surfaces and protects albumen from 
coagulation by their influences. 

The influence of fluid surfaces is evident 
from the fact that in the chemical analysis of 
beer, benzine, benzol, chloroform, etc., may 
be used to coagulate and shake out the beer 

The formation of gas bubbles tends to 
coagulate the dissolved albumen, and this fact 
killed the so-called " Vacuum Fermentation 
Process." The jarring due to transportation 
or even to passing trains may have a deleteri- 
ous effect. A slight trace of acid tends to 
stabilize the albumen as do the tannin and 


resins from the hops, the dextrins from the 
mash and the inorganic colloids of calcium and 
magnesium. A proper balance between the 
dextrin and albumen is necessary for the 
formation of a lasting foam and a desirable 
"body" (Vollmundigkeit). 

In America where beer is served icy cold, 
the chilling produces cloudiness, consequent 
upon a coagulation of albumen. This was 
cleverly overcome by Wallerstein, who in- 
troduced a proteolytic enzyme which increases 
the degree of dispersion of the albumen and 
thus prevents the clouding. 

TANNING. The skins of animals (hide) 
constitute an organized colloid jelly, formed of 
bundles of fine fibrils, about 1 /* in diameter, 
bound together by a cementing material of 
similar chemical composition, which is largely 
removed by the liming and other treatment, 
preceding the tannage proper. 

When the swollen hide is placed in the acid 
tannin solution* (tan liquor), the tannin is 
powerfully adsorbed by the fiber and combines 
with it to form leather. It is still a moot 

* In alkaline solution both the tannin and thejaide are 
negatively charged and no tanning occurs. 


question whether the combination is "phys- 
ical " or "chemical," but since the fixation of 
the tannin follows an adsorption isotherm and 
is reversible in the presence of alkalis, it may 
justly be called a " colloid combination" 
which partakes of the nature of both. The 
positively charged hide and the negatively 
charged tannin mutually coagulate each other. 
Gelatin when neutral and free from electro- 
lytes does not precipitate pure tannin, but in 
acid solution it takes a positive charge and is 
tanned. The tanning process may be aided 
electrically by giving the hide a suitable 
potential, positive in the case of tannin and 
negative in the case of chromium compounds. 

RUBBER. Rubber is made by coagulating 
the milky juice (latex) of various plants. 
Rubber latices are emulsions stabilized by 
protective colloids (proteins or peptones) and 
the nature of the coagulant depends upon the 
nature of the protector. Thus, formaldehyde 
preserves latices whose protectors are proteins, 
but coagulates Kickexia latex by precipitating 
the protective peptones. 

Vulcanization consists of the combination 
of sulphur with rubber. At first the sulphur 


is adsorbed; and then by heating, part of it 
enters into a close combination, probably true 
chemical combination. 

PHOTOGRAPHY. The photographic plate 
owes its sensitiveness to an " emulsion" of 
colloidal silver halides stabilized by a protec- 
tive colloid (gelatin, albumen or collodion). 
The degree of dispersion is controlled by the 
conditions of precipitation of the silver salt 
and the subsequent treatment of the emulsion 
(ripening). The latent image formed upon 
the exposure of the plate to light is probably 
an adsorption compound between colloidal 
silver and the silver halides. 

BOILER SCALE. In addition to containing 
various salts intended to precipitate scale- 
forming ingredients, most formulas for " boiler- 
compounds " and scale-preventing mixtures 
include such substances as glue, dextrin, 
starch, potatoes, tannin, extract of hemlock, 
etc. These colloids undoubtedly prevent the 
formation of hard crystalline scale, either by 
inhibiting to some extent the precipitation of 
the scale-forming salts or by keeping the 
precipitate in an extremely fine non-crystal- 
line condition. 



freshly mixed, cement and mortar contain 
colloidal sols or gels, which gradually coagu- 
late or "set " and bind the crystalline elements 
of the plaster into a coherent whole. 

The setting of the plaster of Paris is delayed 
by glues, gums and other colloidal substances, 
and "retarders " of this character have been 
in use for years. On preparing some micro- 
scope slides with a mixture containing equal 
parts of plaster of Paris and water, to which 
had been added varying proportions of gelatin, 
the following results were observed: 

Per cent 

Time to set 
in minutes. 

Microscopic appearance of slide. 


Characteristic interlacing crystals of calcium 




No true crystals except in a few spots, where 

some colloid-free solution had diffused out. 

Elsewhere aborted sphero-crystals. 



No true crystals. 



No true crystals. 



No true crystals. 


Not set in 

No true crystals. 

48 hours. 


Not set in 

No true crystals. 

48 hours. 

FILTRATION. Successful filtration depends 
upon the use of a septum or filtering medium, 


whose pores or orifices are small enough to 
hold back the particles it is desired to sepa- 
rate from the fluid; or the pores may be- 
come small enough by the deposit upon or in 
them of some of the precipitate, or of some 
added material, such as paper pulp, kieselguhr 
or shredded asbestos. It is, therefore, evident 
that the presence of protective colloids, by 
tending to produce the finely dispersed or 
"hydrosol" condition of the particles, favors 
their passage through the filter. Thus a gold 
hydrosol with particles of 20-30 ju^ and con- 
taining albumen, passed freely through a 
Pukall and a Maassen filter. In the absence 
of the protective albumen, the colloidal gold 
was adsorbed by the filter, gradually clogging 
the pores until the filtrate, at first red, became 
colorless. In technical practice, wherever 
possible, a coagulated precipitate is formed, 
whose large particles are held back with com- 
parative ease. It is very difficult to filter glue 
or gelatin solutions or precipitates formed in 
the presence of protective colloids. 

The successful treatment of sewage, back- 
waters and trade effluents depends largely upon 
the separation from them of colloidal impuri- 


ties by coagulation, adsorption and filtration. 
The old ABC method depended upon the use 
of alum, blood and clay (whence the name) to 
make a coagulum which would carry down 
suspended matter. Ferrous sulphate and lime 
(yielding a coagulum of ferric hydroxid) and 
alum are also used as clarifiers and coagulants. 
Filtration through sand, coke, etc., is made use 
of to adsorb finely dispersed impurities. 

Animal charcoal and fuller's earth decolorize 
sugar and oils respectively, because of their 
powerful adsorptive action. 

CHEMICAL ANALYSIS. The presence of col- 
loids, especially in technical products or solu- 
tions, may lead to grave errors in analysis, so 
that the chemist should destroy them by 
ignition, or else nullify their effects by the 
addition of a sufficient excess of coagulant or 
precipitant. Reversible colloids which are 
frequently referred to under the vague term 
" organic matter " may act: (1) by totally or 
partially preventing the formation of precipi- 
tates, just as tartaric acid and tartrates prevent 
the precipitation of alumina, chromic oxid, and 
ferric oxid (see Yoshimoto, J. S. C. I., 1908, 
27, 952); (2) by preventing the satisfactory 


filtration of the precipitate formed (see Mooers 
and Hampton, J. Am. Chem. Soc., 30, 805); (3) 
by rendering precipitates difficult to wash and 
purify (see Duclaux, J. S. C. L, 1906, 25, 866). 

A few experiments will serve to make clear 
the importance of these remarks. Three 
solutions of lead acetate were taken; to the 
first was added hydrochloric acid which yielded 
a heavy coagulated precipitate; to the second 
was added sodium chlorid (a less highly 
ionized precipitant) which yielded a colloidal 
precipitate of lead chlorid; to the third was 
added, first, a little glue solution and then 
sodium chlorid which in this case gave no 
precipitate at all. 

Again in the presence of glue, silver nitrate 
gives with sodium chlorid only an opalescence 
which passes through filter paper. Even a 
large excess of hydrochloric acid fails to pro- 
duce a precipitate. But upon adding silver ni- 
trate solution to a chlorid solution containing no 
colloid, a copious precipitation occurs at once. 

PHARMACY. Colloids, such as gum arabic, 
Irish moss, tragacanth, etc., are largely used 
in pharmacy in the preparation of emulsions. 
If ferric chlorid be added to gum arabic emul- 


sion of cod liver oil, it coagulates the gum, and 
the oil, no longer protected by the emulso- 
static action of the gum, promptly separates 

Colloidal silver (collargol, argyrol), colloidal 
mercury (hygrol, blue ointment), and col- 
loidal sulphur (ichythol) are largely used 
medicinely. Ferric salts, especially the chlo- 
rid which readily hydrolyzes into the hydrate, 
act as styptics or hemostatics by coagulating 
the blood colloids. The action of disinfectants 
is largely controlled by colloid-chemical factors 
the disinfectants are adsorbed by bacteria, 
and either coagulate their protoplasm or flock 
them out. 

serious error to judge foods upon the basis of 
a bald chemical or calorific analysis. Fat, 
protein, carbohydrate and calories are not 
alone the criteria of food value the physical 
condition of food largely governs its useful- 
ness to the organism. The experiences of 
centuries has taught us the value of "light" 
bread or cake, leavened by yeast or baking 
powder until it presents an enormous surface 
to the digestive juices; unleavened bread was 


eaten only in time of stress, as we learn from 
the Bible. The meats yielded by young 
animals are more juicy and tender than those 
obtained from older animals, because the latter 
are formed from tissues partially dehydrated 
by age. 

The ancient art of cooking involves many 
factors besides mere digestibility and assimila- 
tion; taste, flavor, odor and variety are 
important. Egg albumen when cooked is 
probably more slowly absorbed and loses its 
species-specificity; therefore, some people who 
have an idiosyncracy against raw eggs can 
eat cooked eggs. Cream is an emulsion of fat 
in an aqueous medium and wets paper; butter 
is an emulsion of water in a fatty medium and 
greases paper. 

which occur on almost all physiological proc- 
esses are remarkable not only because of 
their very profound nature, but also because 
they are produced at comparatively low tem- 
peratures and in the presence of very dilute 
reagents. The living organism disintegrates 
proteins, oxidizes carbohydrates and with the 
same apparent ease synthesizes substances of 


great complexity. Powerful reagents and high 
temperatures, which would be destructive to 
life, are necessary to bring about changes of 
this character under ordinary laboratory con- 

The body and plant colloids (biocolloids) 
consist of carbohydrates (starch, cellulose, 
glycogen), proteins (plant and animal al- 
bumins), and lipoids (lecithin, cholesterin, fats 
and oils). Each tissue has a normal turgor or 
state of swelling which is greatly influenced 
by acids, alkalis and salts. The swelling and 
shrinking of tissues, together with their selective 
adsorption and the differential diffusion of 
solutions through them, account for or accom- 
pany many physiological phenomena, both 
normal and pathological. Thus, fibrin and 
gelatin swell much more in very dilute acid 
than in distilled water, but the swelling is 
depressed by salts. Fibrin is so sensitive that 
it swells in the presence of traces of acid quite 
undetectable by ordinary indicators, such as 
litmus; in fact fibrin itself is a most sensitive 

* Though the normal H ion concentration of the blood is 
0.37 X 10~ 7 , a concentration of 1.00 X 10~ 7 nH represents 
an advanced acid intoxication. 


Local accumulation of acid in the organism 
may cause swelling (edema) ; for example, in- 
sect stings, which may be imitated by stinging 
gelatin with a needle dipped in acid. If acid 
accumulates in an organ with a rigid capsule 
(eye or kidney), the swelling tends to establish 
a vicious circle (glaucoma, nephritis) by com- 
pressing the blood vessels and cutting down 
the alkaline blood stream, which is unable to 
wash out the acids (mainly CO 2 ) formed by 
living protoplasm. 

If the oxidation processes of the body are 
normal, the hydrogen in foods is oxidized 
mainly to water and the carbon mainly to 
carbonic acid a gaseous acid which is ex- 
haled without demanding fixed alkali or pro- 
tein of the organism for its elimination. It 
would require nearly two pounds of pure caus- 
tic soda to neutralize the acidity produced daily 
by an average man. In the case of pathologi- 
cal oxidation, however, other non-volatile acids 
are formed and a condition called "acidosis " 
may arise, which is in reality a diminished 
alkalinity, recognizable by the fact that an 
abnormally large quantity of bicarbonate of 
soda is needed to render the urine alkaline. 


These acids may cause disturbances of the 
body colloids, disease and even death. In fact, 
throughout life there is a gradual syneresis of 
the biocolloids accompanied by visible shrink- 
ing and loss of water compare the chubby 
hand of a child with that of an old man. In 
plants an analogous process occurs in lignifi- 

DIGESTION. The digestive process is pre- 
liminary to the actual adsorption and use of 
food by the organism, and has for its object 
the modification or change of the ingested food 
into such forms or such substances as may be 
absorbed in the lower part of the digestive 
tube. To have a correct understanding of the 
absorption of the products of digestion, we 
must bear in mind the fact that the walls of the 
digestive tract act as semipermeable colloid 
membranes and that absorption involves dif- 
fusion into or through these membranes or 
their constituent cells. Substances in crystal- 
loidal solution, and colloidal sols whose par- 
ticles are sufficiently small, represent then the 
two classes of digestion products which are 
diffusible and therefore absorbable. 

Food as ingested consists mainly of sub- 


stances that may be grouped into two classes: 

1. Crystalloids such as water, sugars, 
sodium chlorid, etc. 

2. Colloids such as starch, proteins, emul- 
sions, etc. 

The crystalloids in foods are usually absorbed 
directly, although sucrose, for example, under- 
goes inversion. The colloids, as a rule, are not 
directly absorbable, and, for the most part, 
digestion consists in^the disintegration of the 
colloidal complexes of the food, so that they 
can actually diffuse into the organism and 
there undergo further changes. Colloidal gels 
or even sols whose particles are of large size 
are, practically speaking, non-diffusible, and 
must, therefore, be reduced to a more finely 
dispersed state. 

Investigation has demonstrated that the 
high efficiency of the digestive juices is mainly 
due to small quantities of certain colloidal 
substances called enzymes (such as ptyalin, 
pepsin and pancreatin) which act as catalyzers, 
enormously hastening reactions which would 
otherwise proceed so slowly that, practically 
speaking, they would not occur at all. The 
enzymes appear to act by forming with the 


substrate a combination of unstable character, 
which breaks down and liberates the enzyme 
again to continue the operation. Recently 
W. M. Bayliss, in his interesting monograph 
on "The Nature of Enzyme Action," has 
shown that in all probability "the compound 
of enzyme and substrate, generally regarded as 
preliminary to action, is in the nature of a 
colloidal adsorption compound." Anyone who 
has seen in the ultramicroscope the extremely 
active motion of the individual particles in 
colloidal solutions, can readily imagine the 
terrific bombardment a substance must un- 
dergo when a colloid enzyme is concentrated 
on its surface by adsorption, and indeed it 
seems probable that enzymes actually produce 
their effects by virtue of their specific surface 
actions and the motion of their particles. 

In order to find out if this idea could be 
verified by actual observation, the author 
watched under the ultramicroscope the action 
of diastase upon potato starch grains and the 
action of pepsin upon coagulated egg albumen. 

In the first case, actively moving ultra- 
microns in the diastase solution gradually 
accumulated about the starch grains, which 


after a time showed a ragged and gnawed 
margin. While the adsorption and motion of 
the larger ultramicrons was all that could be 
followed, the bright appearance of the field 
indicated that more numerous finer particles 
were present, and some apparently of inter- 
mediate size were seen. 

For observations on albumen there was used 
a dilute solution of white of an egg which has 
been heated nearly to boiling. It was opales- 
cent and in the ultra apparatus exhibited a field 
full of bright and rapidly moving ultramicrons. 
Upon allowing a droplet of essence of pepsin 
(Fairchild's, containing 15 per cent of alcohol 
by weight) to diffuse in, an immediate coagu- 
lation occurred, the particles clumping into 
very large masses. A droplet of decinormal 
hydrochloric acid was then allowed to diffuse 
in, whereupon the large masses broke up in 
small groups and single ultramicrons, which 
once more resumed their original motion. 
Soon, however, the albumen particles began 
to grow smaller and disappear, the field all the 
while becoming brighter and brighter, indicat- 
ing the concommitant appearance of smaller 
ultramicrons or amicrons. In vitro the addi- 


tion of the pepsin to the opalescent albumen 
solution caused it to clear gradually, even at 
room temperature. 

Enzymes are inactivated to a greater or less 
extent by shaking, heating, electrolytes, etc., 
all of which, as is well known, cause the 
coagulation of colloidal solutions and a result- 
ing decrease in the activity of the motion of 
their constituent particles. Another feature 
of interest is that the action of enzymes is 
reversible, a fact that does not come much 
into evidence because of the dilution and 
removal by diffusion of the products formed. 
In cells, tissues and organs, however, changes 
of concentration again occur and synthetic 
processes may result. 

One principle of colloid chemistry is of the 
utmost importance in digestion, namely: the 
protective action of reversible colloids, which 
stabilize or protect from coagulation irrevers- 
ible or unstable colloids. Mucin and analo- 
gous colloidal substances undoubtedly have a 
function of this character, which may in some 
cases account for the variance between the 
action of natural and artificial digestive juices. 
The effects of colloidal protection are in 


evidence in almost all physiological reactions 
and processes, and it is indeed extremely 
doubtful if there ever occurs in vivo any 
chemical reaction which is not greatly in- 
fluenced by the colloids always present. 

These are largely affected by the swelling and 
shrinking of the body colloids and by selective 
adsorption and diff erential diffusion. It must 
be remembered that the blood is in reality a 
circulating fluid colloid, whose attraction for 
water is greater in the "acid " or venous con- 
dition, than it is in the " alkaline " or arterial 
condition. Tissues and organs well supplied 
with venous blood tend to adsorb water 
(intestine); whereas those well supplied with 
arterial blood tend to give up (secrete or ex- 
crete) water (kidney); and as the blood is 
passing in a continuous stream, the process 
continues as long as the water supply permits 
and until the blood is in equilibrium with the 
other tissues.* 

* The functioning of organs is largely controlled by nervous 
influences. Thus a sudden nervous shock may by vaso-dilation 
send an excessive supply of arterial blood through the mes- 
enteric arteries (an "internal blush"), and result in a secre- 
tion of fluid into the intestine (nervous diarrhea). 


Conditions which decrease the capacity of 
the blood and tissues to hold water (diuretics, 
hyperglucemia and acidosis in diabetes) natu- 
rally result in the elimination of the excess or 
"free " water (polyuria, diarrhea). 

Minute quantities of acid increase the swell- 
ing capacity of colloids, which quickly reaches 
a maximum; after which increasing acidity 
causes shrinking. Neutral salts oppose the ac- 
tion of acids apparently by driving back the 
ionization of the acid and thereby reducing the 
H-ion concentration which is the controlling 

The action of selective adsorption and 
differential diffusion in effecting secretion and 
excretion must be at once manifest. Easily 
hydrolyzable compounds may be thus split up 
in the body, and yield secretions of acid nature 
like the gastric juice, or of alkaline nature like 
the pancreatic juice, depending upon the 
structure of the organ, the location of its 
cavity and of its afferent and efferent vessels. 
Individual compounds in the blood stream or 
other body juices may also be selectively 
diffused out, concentrated or separated from 
other accompanying substances. By selective 



adsorption, circulating substances may be 
fixed and taken from the circulation; in fact, 
poisons are usually taken up selectively by 
certain organs and tissues. 

An insight into the mechanism of body 
processes may be obtained by considering the 



f|l<~7iy/ of Renal Artery 
FIG. 1. Glomerular structure.* 

functioning of the kidney (see Fig. 1). The 
Malpighian tufts are plentifully supplied 
with arterial blood having "free water/' and 

* From Dr. J. G. M. Bullowa's translation of Bechhold's 
"Colloids in Biology and Medicine," D. Van Nostrand Co., 


under the pulsating pressure* of the blood 
stream, they ultrafilter off a very dilute but 
copious blood ultra-filtrate into the long con- 
voluted tubules. The tubules, however, are 
plentifully supplied with venous blood, which 
is unsaturated with water and which there- 
fore reabsorbs most of the water together 
with some of the dissolved substances contained 
in the preliminary excretion ; so that there drips 
into the pelvis of the kidney a concentrated 
urine having in solution many of the substances 
found hi the blood, but in a much higher 
concentration. Bechhold estimates that the 
average of two liters of urine voided daily by an 
average man, represents a preliminary excretion 
of fifty liters, of which forty-eight are reab- 
sorbed within the kidney itself. 

In plants, differential diffusion and selective 
adsorption seem to be intimately bound up 
with growth and the circulation of the sap. 
The plant tissues are mainly colloidal gels or 
finely integrated structures, and as the sap 
circulates or diffuses through them, each tissue 

* Since the vas defferens has a smaller lumen than the vas 
efferens, a " back pressure " is created within the Malpighian 


selectively adsorbs and elaborates certain 
particular constituents. Thus with the potato 
and tapioca plants the starch forming sub- 
stances are fixed in the roots; with the sago 
palm they are fixed in the stem pith ; and with 
cereal grains, in the seeds. As long as the 
adsorptive tissues are unsaturated or are 
multiplied, so long can growth continue, the 
stem and branches taking up the substances 
required for the upward growth, and the root 
taking up those required for the downward 

When we consider the great variety of bio- 
colloids and their susceptibility to changes of 
structure and diffusive or adsorptive capacity, 
we can easily understand the almost infinite 
number of reactions that may go on within 
their recesses, as they swing the balance of the 
law of mass action over particles reduced to 
a reactive degree of subdivision. 


The following are some of the more important standards of 


H. BECHHOLD, "Colloids in Biology and Medicine" (trans. 

by Dr. J. G. M. Bullowa). 1919. 

E. F. BURTON, "The Physical Properties of Colloidal Solu- 
tions." 1916. 
M. H. FISCHER, "(Edema and Nephritis." 1915. "Fats 

and Fatty Degeneration." 1917. 
Wo. OSTWALD, "Theoretical and Applied Colloid Chemistry" 

(trans, by Dr. M. H. Fischer). 1917. 
Wo. OSTWALD, "An Introduction to Theoretical and Applied 

Chemistry" (trans, by Dr. H. M. Fischer). 1916. 
ZSIGMONDY, "Colloids and the Ultramicroscope " (trans, by 

J. Alexander). 1909. 
ZSIGMONDY, "Chemistry of Colloids" (trans, by E. Spear). 



COTTON ET MOUTON, "Les Ultramicroscopes et les objets 

Ultramicroscopiques." 1906. 
PAUL GASTOU, " L'Ultramicroscope dans le Diagnostic Cli- 

nique et les Recherches de Laboratoire." 1916. 
PERRIN, Numerous Journal Articles. 




ARTHUR IV!ULLER, "Allgemeine Chemie der Kolloide." 1907. 
Wo. OSTWALD, "Grundriss der Kolloidchemie." 1909. 
THE SVEDBERG, "Herstellung Kolloider Losungen." 1909. 
FREUNDLICH, "Kapillarchemie." 1909. 
VAN BEMMELEN, "Die Absorption." 1911. 

The "Zeitschrift fur Chemie und Industrie der Kolloide 
(Kolloid-Zeitschrift)" and " Kolloidchemische Beihefte," 
published by Wo. Ostwald, are mines of information, con- 
taining both original articles and references. 

Abstracts of, or references to practically all current articles 
and books on Colloid Chemistry are to be found under the 
division "Physical Chemistry "of "Chemical Abstracts," 
published by the American Chemical Society. Furthermore, 
in the books above referred to, especially Burton, are to be 
found numerous valuable references. 


ALEXANDER, J., 50, 57, 85. 
ATTERBERG, 41, 44. 
BECHHOLD, 26, 81, 85. 
BEHRE, 16. 
BEILBY, 49. 
BETTS, A., 47. 

BlLTZ, 16. 

BREDIG, 10. 
BULLOWA, 57, 81, 85. 

BUNSEN, 16. 

BURTON, 33, 85. 
COEHN, 30. 
COTTON, 23, 85. 




FISCHER, M. F., 85. 
GASTOU, P., 85. 
GRAHAM, T., 1, 25, 33. 
HARDY, 10, 31, 32. 
HERSEY, 49. 
JACKSON, H., 53. 


LODGE, O., 33. 


MAYER, 54. 
MOOERS, 69. 
MOUTON, 23, 85. 
MUELLER, 25, 47, 85. 

85, 86. 
PATTEN, 44. 
PERRIN, 85. 
PUTZ, 49. 
RAFFO, M., 14. 


SlEDENTQPF, 11, 19, 23. 

SPEAR, E., 85. 
SPRING, W., 14. 
VON WEIMARN, P. P., 10. 
WUST, 49. 
ZSIGMONDY, 7, 10, 11, 16, 17, 
25, 48, 55, 85. 


Absorption, 19, 86. 

Acidosis, 73. 

Adsorption, 16, 50, 82. 

Agriculture, 43. 

Amicrons, 22. 

Astronomy, 36. 

Atmosphere, 40. 

Boiler scale, 65. 

Brewing, 61. 

Brownian motion, 8. 

Cardioid Condenser, 23. 

Cement, 66. 

Ceramics, 42. 

Chemical Analysis, 68. 

Clay, 42. 

Coehn's law, 30. 

Colloid Chemistry, Applica- 
tions of, 36. 
definition of, 6. 

Colloids, Classification of, 10. 

Colloidal protection, 24, 78. 

Comets, 37. 

Confectionary, 61. 

Cosmic dust, 37. 

Deflocculation, 35. 

Dialysis, 25. 

Diffusion, 25, 27. 

Digestion, 74. 

Dimensions of colloidal par- 
ticles, 7. 22. 


Dispersoids, 10. 

Dyeing, 50. 

Edema, 73. 

Electric charge of colloidal 

particles, 30. 
Electrophoresis, 30. 
Electroplating, 47. 
Emulsoids, 12. 
Enzymes, 75, 78. 
Excretion, 79. 
Fatty Degeneration, 85. 
Filtration, 66. 
Foods, 70. 
Geology, 41. 
Glaucoma, 73. 
Gold number, 32. 
Hydrophile colloids, 12. 
Hydrophobe colloids, 12. 
Ice cream, 60. 
Irreversible colloids, 10, 12. 
Isoelectric point, 32. 
Kidney, 81. 
Lyophile colloids, 12. 
Lyophobe colloids, 12. 
Metallurgy, 48. 
Meteorology, 39. 

Micron, (M) = YQQQ mm< 
Migration of Colloids, 31. 


Milk, 56. Radius of molecular attrac- 

, N 1 tion, 14. 

Millimicron, M = 1^0 * Reversible coUoids, 10. 

1 Rubber, 64. 

~ 1,000,000 m Schutz kolloide, 44. 

Mineralogy, 41. Secretion, 79. 

Mortar, 66. Soap, 52. 

Nephritis, 73, 85. Soils, 43. 

Pathology, 71. Solution, 7. 

Pectization, 31. Submicrons, 22. 

Pedesis, 8, 53. Suspensions, 7. 

Peptization, 31, 33. Tanning, 63. 

Pharmacy, 69. Tyndall effect, 20. 

Photography, 65. Ultrafiltration, 25. 

Physiology, 71. Ultramicrons, 22. 

Plaster, 66. Ultramicroscope, 17. 

Purple of Cassius, 16- Weather, 39. 


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Suspensio n-s- 

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Classification of Colloidal Solutions 

according to the size of the particles contained in them and 
according to their behavior upon desiccation. 




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OCT27 1939 

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